This invention relates to a method for applying a continuous protective coating to the surfaces of individual phosphor particles. The method involves chemical vapor deposition of the protective coating on individual particles of a phosphor powder while the particles are suspended in a fluidized bed.
U.S. Pat. No. 4,585,673 ('673 patent) to Sigai which is incorporated by reference herein, describes a relatively complex gas phase technique for the deposition of protective coatings on individual phosphor particles by chemical vapor deposition while the particles are suspended in a fluidized bed. The fluidized particles are exposed to a vaporized precursor material at a temperature that is below that at which a precursor material decomposes. After the particles have been enveloped by the precursor material at a first temperature, the absorbed precursor is reacted to form a continuous protective coating on the surfaces of the individual particles at a second higher temperature. In a preferred version of this technique, the volatizable coating precursor is vaporized into an inert carrier gas which is subsequently passed through a phosphor powder to form a coating precursor material. The fluidized bed is maintained in a temperature gradient ranging from a lowest temperature to a highest temperature, the lowest temperature being less than the decomposition temperature of the coating precursor material, and the highest temperature being at least sufficient to react the precursor with an oxidizing gas to form the desired coating. The oxidizing gas is passed into the fluidized bed separately from the vapor containing carrier gas.
In the examples cited by the 673 patent, the coating precursor is trimethylaluminum (TMA) and the oxidizing gas is molecular oxygen. However, the patent also contains claims concerning the use of oxygen containing coating precursor materials (see claims 10, 22, and 25). An example of such an oxygen containing precursor is aluminum isopropoxide (AIP). AIP has frequently been used to form alumina coatings on semiconductor substrates via chemical vapor deposition as well as on metal glass or ceramic substrates. In U.S. Pat. No. 3,408,223 a process in which phosphor particles are exposed to a gaseous medium including oxygen and aluminum oxide formed by heating a vaporized mixture of AIP and oxygen is disclosed. The oxide materials are condensed on the surfaces of the phosphor particles to form surface barrier layers suitable for modifying the electron beam energy threshold of the material. In the process, the particles are passed downwardly through the heated gaseous medium, preferably along a generally spiral path, recirculating repetitively through the gaseous medium.
There are several reasons why AIP might be preferred over TMA as a coating precursor. Perhaps the most compelling of these is the safety factor. AIP is a solid material at ambient temperatures and pressures rather than a volatile liquid such as TMA. Further, although AIP is flammable (as are the vast majority of organic and organometallic compounds), it is not pyrophoric. In contrast TMA (along with other aluminum alkyls) bursts into flame upon coming into contact with air or moisture, producing smoke containing finely divided aluminum oxides and alkoxides. AIP owes its nonpyrophoricity mainly to its molecular structure which contains three oxygen atoms bonded to a central aluminum atom. Thus, the aluminum atom is already in an oxidized state. Besides making AIP almost infinitely safer to handle and use than TMA. the presence of the aluminum-oxygen bonds also provides a means to the formation of aluminum oxide, via thermal decomposition, without requiring a source of external oxygen. Finally, the cost of AIP is only about 2% of the cost of TMA, an obvious commercial advantage.
However, there are disadvantages associated with the use of AIP as a coating precursor in the process described in the '673 patent One relatively minor disadvantage is the fact that the AIP bubbler through which the nitrogen carrier gas flows on its way to the entrance to the fluidized bed must be maintained at a temperature in the vicinity of 150.degree. C. to achieve AIP vapor pressures and transport rates comparable to those obtained with TMA in a 30.degree. C. temperature bubbler. This means that the line exiting the bubbler leading to the fluidized bed reactor, must be maintained at a temperature of approximately 150.degree. C. to prevent condensation of the AIP vapors within the heated line.
The second and more serious disadvantage derives from the decomposition kinetics of AIP compared with those of TMA. Both TMA and AIP begin thermally decomposing at temperatures between 150.degree. and 200.degree. C. With TMA. the main products of decomposition are aluminum carbide and methane as shown below. EQU 4Al(CH.sub.3).sub.3 .fwdarw.Al.sub.4 C.sub.3 +9CH.sub.4 (1)
The aluminum carbide forms as a dense, dark colored deposit.
AIP, on the other hand, decomposes to form aluminum oxide, isopropanol, and propane as shown below. EQU 2Al(i--OC.sub.3 H.sub.7).sub.3 .fwdarw.Al.sub.2 O.sub.3 +3C.sub.3 H.sub.7 OH+3C.sub.3 H.sub.6 (2)
The aluminum oxide forms as a voluminous white deposit. The thermal decomposition of AIP is an undesirable reaction and must be avoided if a controlled, predictable phosphor coating process is to be achieved.
If the thermal decomposition of TMA does occur to a limited event products are formed, a dense solid and a low molecular weight gas, which do not interfere significantly with the phosphor coating reaction. In contrast, if the thermal decomposition of AIP occurs, products are formed, a voluminous sticky solid and comparatively high molecular weight alcohols and olefins, which at the very least increase the effective viscosity of the fluidized bed and which at worst can clog the pores of the gas distributor, essentially shutting down the fluidized bed reactor.
Moreover, a similar situation exists with respect to the coating reaction itself. With both TMA and AIP. complete oxidation of the coating precursor produces aluminum oxide, carbon dioxide, and water. However, the incomplete oxidation of the AIP precursor produces a mixture of partially oxidized methane, ethane, and propane derivatives, while the incomplete oxidation of TMA produces only a relatively small quantity of methane derivatives. Thus, at any point in the coating process, there is a much higher concentration of comparatively high molecular weight partially oxidized hydrocarbons in the fluidized phosphor bed with AIP as the coating precursor than there ever is with TMA as the precursor. Such hydrocarbon substances are absorbed upon the surfaces of the phosphor particles, leading to increased cohesive forces between these particles, thereby increasing the effective viscosity of the fluidized bed.
Another drawback to using AIP as a precursor is that residual carbon concentrations tend to be higher on the alumina coated phosphor particles than when using TMA as a precursor. Higher carbon concentrations in the conformal alumina coating reduce the phosphor brightness of the annealed particles. Increased carbon concentrations in the alumina coating also lead to increased phosphor body-color and reduced luminescent efficiency. The resulting phosphor is therefore not a commercially viable product for lamp production.
Thus, in order to employ AIP as a coating precursor in the fluidized bed reactor, the decomposition of the precursor must be prevented, at least until the precursor contacts the phosphor powder within the fluidized bed. In addition, the fluidized bed must be operated in a manner so as to counteract the tendency towards particle agglomeration that results from the presence within the bed of a relatively high concentration of organic byproducts and intermediates and results in high carbon concentrations in the alumina coatings on the phosphor particles.
However, these process design goals are largely in opposition to one another. On one hand, we seek to maximize fluidization efficiency so as to prevent the breakdown or clogging of the bed due to the buildup of organic byproducts and intermediates. On the other hand, the more efficiently the bed is fluidized, the more nearly isothermal it becomes. Moreover, due to the extremely efficient heat transfer between the fluidized particles and the walls of the container, the temperature of the surface of the gas distributor approaches that of the fluidized bed. Since the coating process operates most efficiently with bed temperatures above about 400.degree. C., the more efficiently the bed fluidizes, the greater the chance of precursor thermal decomposition as it flows along with the carrier gas through the plenum which underlies and supports the gas distributor and on through the fine-pored distributor itself. Thus, in order to realize the substantial advantages of AIP over TMA as a coating precursor, techniques must be found to maintain efficient fluidization within the fluidized bed, despite the presence within the bed of organic materials which tend to increase particle agglomeration and stickiness, while at the same time ensuring that the AIP containing carrier gas does not contact a surface temperature in excess of about 180.degree. C. prior to entering the fluidized bed. This must be accomplished despite the fact that the bed must be maintained at temperatures in excess of approximately 400.degree. C. to achieve nearly complete oxidation of the organic byproducts of the coating reaction. The present invention effectively solves these problems and describes a method in which alumina coatings are applied to phosphor particles in a fluidized bed using AIP as a coating precursor.